This invention relates to a method of recognizing fingerprints, and more particularly to a method of smoothing a ridge direction pattern when the flow of the ridge lines is used for precise interpretation of fingerprints.
A method of smoothing a ridge direction pattern has been disclosed, for example, on "Fingerprint Pattern Classification" by Masahiro Kawagoe et al in Pattern Recognition Vol. 17, No. 3, pp. 295-303, 1984, on "A Restoration Algorithm of Fingerprint Images"by Osamu Nakamura et al in Treatise of Japanese Electronics and Communication Vol. J68, No. 5, pp 1133-1140, 1985, and on "Automated Fingerprint Identification by Minutia-Network Feature" by Ko Asai et al in Treatise of Japanese Electronics, Information and Communication Vol. J72-D-II, pp724-732, 1989.
In these heretofore known systems, a fingerprint is input, for example, as a 512.times.512 binary image(dot matrix) and converted into an array of direction patterns composed of 32.times.32 subregions, each corresponding to a 16.times.16 pixel area.
The direction in each subregion is quantized in eight different levels, that is, 0, .pi./8, 2.pi./8, 3.pi./8, 4.pi./8, 5.pi./8, 6.pi./8, 7.pi./8 and is labeled by d from d=1 to d=8, d=1 corresponding to 0 level and d=7 corresponding to 7.pi./8 level. In addition to these eight levels, d=0 is labeled to a subregion where no ridge line is contained or where the direction pattern is not defined.
In the conversion into these nine labels, a quantized direction having the highest probability in a subregion is labeled to the subregion, and its probability is called an initial probability of the initial direction.
Then, the probabilities of the directions of all the subregions are repeatedly innovated by probability relaxation process in which a renovated probability is expressed by ##EQU1## P.sub.j.sup.k (d) is the probability of direction d at position j in the kth iteration,
j,(j') is the subregions position, PA1 a is the extent of the subregions having influence on a subregion concerned, PA1 W.sub.jj, is a weight of influence from a subregion at point j' to a subregion at point j, PA1 .gamma..sub.jj,(d,d') is the compatibility coefficient, for example =1 when d is parallel to d', and =-1 when d is perpendicular to d'.
At each iteration of the probability relaxation process, the direction having the highest probability in a subregion may change and the label of the subregion is changed.
After a certain number of iterations of the process, the label determined by the final iteration is taken as a smoothed label of the subregion.
Although the initial probability of a label for each subregion is determined at the initial conversion of the fingerprint, the label of a subregion can be changed at each iteration of the relaxation process without a constraint from the initial probability. Thus, there is no guarantee of convergence of a direction of a subregion, and an iteration of the step does not necessarily improve the smoothness of the ridge line pattern. Therefore, the operator must decide the termination of iteration by trial and error, observing the results on the way, and it is difficult to obtain a reliable ridge line pattern by this relaxation process.
And in this relaxation process, a label of low probability(poor reliability) is treated equally as a label of high probability(good reliability). As a result, a label having a high initial probability can be changed to a wrong direction by an influence from a label having a low probability.